260 PROBLEMS OF FERTILIZATION 



about fifteen seconds later, it and is completed in 

 fifteen to twenty seconds more. In this case the entire 

 egg has already become impermeable to sperm at the 

 moment that membrane elevation has begun. There 

 is in fact a '"wave of negativity' that sweeps over the 

 egg preceding the wave of membrane elevation, so that 

 any point on the surface becomes impermeable to sperm 

 (unfertilizable) some time before membrane elevation 

 begins. The wave of negativity dates from the time 

 that the tip of the sperm head has entered the cyto- 

 plasm. The so-called fertilization cone arises after 

 penetration of the spermatozoon. 



The principles of polyspermic fertilization seem to 

 the writer, also, to furnish a demonstration of the point 

 in question. With a perfectly fresh lot of eggs one 

 can raise the percentage of polyspermy by increasing 

 the concentration of the sperm suspension. Since the 

 egg becomes unfertilizable at any point where the ferti- 

 lization reaction has begun, we would expect that poly- 

 spermy would increase with sperm concentration if there 

 were an appreciable time interval in the spread of the 

 reaction from a point, and this is what we observe. 

 With a given sperm concentration also the proportion 

 of polyspermic eggs increases with weakening of the 

 eggs either by action of reagents or by staling. The 

 obvious implication here is that in weakened eggs 

 the cortical change is propagated more slowly. It is 

 noteworthy in this connection that normal polyspermy 

 occurs only in eggs with considerable yolk content and 

 hence large surface. 



The conclusion that the cortical change is propa- 

 gated from the point of fertilization at a rapid but 



